I did it on my server with Debian 7.
It works perfectly except for Horde and SquirrelMail.
Indeed, I successfully created an email account with my domain name in ISPConfig. Then, with it I can access to Horde and SquirrelMail.
I also configured a rule in ISPConfig to transfer my Gmail account to this email account I created previously.

But, when I check my emails, there is nothing in Horde and in SquirrelMail. I prefer to use Horde but anyway I've the same issue also with SquirrelMail.

I open the ports in ISPConfig and it seems also correct on my Debian IPTABLES config.

A last check in ISPConfig show me that I use an Email quota of 1.1150Kb for this email account and with 772Mb transfered this month.
Then, where are those emails?

For me the domain name MX redirection was correctly configured because I didn't receive email post errors when I try to send an email to this account.